The effect of hydration state and energy balance on innate immunity of a desert reptile

Abstract

Introduction: Immune function is a vital physiological process that is often suppressed during times of resource scarcity due to investments in other physiological systems. While energy is the typical currency that has been examined in such trade-offs, limitations of other resources may similarly lead to trade-offs that affect immune function. Specifically, water is a critical resource with profound implications for organismal ecology, yet its availability can fluctuate at local, regional, and even global levels. Despite this, the effect of osmotic state on immune function has received little attention.

Results: Using agglutination and lysis assays as measures of an organism's plasma concentration of natural antibodies and capacity for foreign cell destruction, respectively, we tested the independent effects of osmotic state, digestive state, and energy balance on innate immune function in free-ranging and laboratory populations of the Gila monster, Heloderma suspectum. This desert-dwelling lizard experiences dehydration and energy resource fluctuations on a seasonal basis. Dehydration was expected to decrease innate immune function, yet we found that dehydration increased lysis and agglutination abilities in both lab and field studies, a relationship that was not simply an effect of an increased concentration of immune molecules. Laboratory-based differences in digestive state were not associated with lysis or agglutination metrics, although in our field population, a loss of fat stores was correlated with an increase in lysis.

Conclusions: Depending on the life history of an organism, osmotic state may have a greater influence on immune function than energy availability. Thus, consideration of osmotic state as a factor influencing immune function will likely improve our understanding of ecoimmunology and the disease dynamics of a wide range of species.

Figure 1

Agglutination and lysis capacities of free-ranging Gila monsters. Agglutination and lysis scores as a function of (A, B) plasma osmolality, (C, D) month of sampling, and (E, F) change in tail volume in free-ranging Gila monsters. Individuals with a greater plasma osmolality (i.e., more dehydrated) had greater agglutination and lysis scores. Change in tail volume (a measure of energy balance) was negatively related to lysis (F_1,21 = 7.74, P = 0.011), but after removing a gravid female from analysis, this was non-significant (F_1,18 = 3.76, P = 0.068), a change denoted with a dashed line. Change in tail volume had no significant relation (F_1,21 = 3.61, P = 0.071) to agglutination. There was no effect of month on either agglutination or lysis ability (all P > 0.05). No individual was disproportionately influential (all Cook’s D < 4/n) in these analyses. Raw data are presented, uncorrected for individual. LSMeans with error bars (SE) from the mixed model are presented for month of sampling.

Figure 2

Agglutination and lysis capacities of captive Gila monsters as a function of hydration state. Individuals had more robust (A) agglutination and (B) lysis scores as they dehydrated, with immune function decreasing to (for agglutination) and even beyond (for lysis) baseline samples post-drinking. Here, hydration state is defined as: hydrated (270–300 mOsm); moderately dehydrated (305–335 mOsm); severely dehydrated (340+ mOsm), 1 day post-drinking (285–295 mOsm); 2 days post-drinking (255–285 mOsm). Groups that share the same letter have LSMeans that are not statistically different (P > 0.05). Bars represent LSMeans, with error bars showing SE.

Figure 3

Effects of dehydration and dilution on agglutination and lysis capacities of captive Gila monsters. (A) Agglutination and (B) lysis scores of plasma from captive Gila monsters tested in both hydrated and dehydrated states, as well as plasma from Gila monsters in the dehydrated state that was diluted with nanopure water by 16 to 23% to match the osmolality of the individual’s hydrated plasma sample. The hydrated state samples had significantly lower agglutination and lysis scores relative to both non-manipulated and diluted dehydrated samples. Groups that share the same letter have LSMeans that are not statistically different (P > 0.05). Bars represent LSMeans, with error bars showing SE.

Figure 4

Effects of digestive state on agglutination and lysis capacities. (A) Agglutination and (B) lysis scores from captive Gila monsters as a function of digestive state. Individuals were given a meal (approximately 20% of body mass) immediately after blood collection on day 0. Neither agglutination nor lysis scores significantly varied (P < 0.05) over the duration of meal absorption. Bars show LSMeans and error bars represent SE.

Figure 5

Degradation of agglutination and lysis capacities over time. Number of days in the refrigerator prior to plasma separation and freezing significantly reduced both (A) agglutination and (B) lysis scores. Groups that share the same letter have LSMeans that are not statistically different (all P < 0.05), although agglutination scores on days 0 and 2 approached significance (P = 0.058). Bars show LSMeans and error bars represent SE.